1. Field of the Invention
This invention relates to an actuator for an internal shoe drum brake, the actuator including means for alternative fluid pressure and mechanical actuation of the brake.
2. Description of the Invention
Various forms of such a combined actuator are already known, the most common combining a hydraulic cylinder and a mechanical wedge device for effecting respectively the two alternative forms of actuation. One of these conventional arrangements includes a wedge device disposed between tappets sliding in respective bores in the actuator housing, the wedge being operable via a pair of rollers to move the tappets outwardly of the housing to actuate a pair of brake shoes. One of the tappets is split along a diametrical plane to form a pair of pistons with a hydraulic chamber between them to permit the pistons to be urged in opposed directions by application of hydraulic pressure, one of the pistons acting to expand the immediately adjacent shoe and the other acting on the other tappet to expand the other shoe. The wedge and roller assembly is capable of limited lateral movement to accommodate this.
Automatic adjustment is a common requirement in present day actuators and because of the generally unfavorable environment in which the actuators have to operate, the adjuster is commonly located internally of the actuator and thereby protected from the environment. It is usually desirable for the tappets to be adjustable independently of each other so that each adjusts only in response to excess movement of the shoe which it actuates. However, with the split-tappet arrangement described above, this requirement has hitherto been impossible to fulfil and an object of the invention is to provide a brake actuator which includes both mechanical wedge and hydraulic actuation means, of which the latter includes a split tappet.